1. Field of the Invention
The present invention relates to an IC socket for detachably accommodating and holding an electrical part such as an IC package and for electrically connecting the IC package to a printed circuit board.
2. Prior Art
In a known art, for example, as shown in FIG. 10 to FIG. 12, an IC socket of this kind is disclosed in Japanese laid-open patent 242977/1999.
As shown in FIG. 10, the IC socket 11 in the known art is structured to be mounted on a printed circuit board 12 and an IC package 13 is mounted and held on the IC socket 11, thereby the IC package 13 is electrically connected to the printed circuit board 12.
The IC socket 11 has a base plate 14, a tab film 15, a ball guide 16 and a pressing jig 17, in this order from bottom to top, and these members are structured to be detachably fixed together by a bolt 18 and a nut 19.
The base plate 14, as shown in FIG. 10, is formed to have a plate like quadrangular shape and has a peripheral portions at four side portions of the base plate 14. A plurality of round pins 14a is disposed at the four sides peripheral portions in two rows. At an inner side of the rows of the round pins, a total of 4 bolt holes 14b and positioning holes 14c are formed. An elastic member 14d made from a silicone rubber is disposed at a center portion of the base plate 14. As shown in FIGS. 10 and 11, the round pin 14a is arranged to penetrate vertically through the base plate 14 and has a concave fitting portion 14e at its upper portion and an inserting portion 14f at its lower portion protruding downward from a lower surface of the base plate 14. The inserting portion 14f is designed to insert into a through hole 12a of the printed circuit board 12 to establish electrical connection between the round pin 14a and the printed circuit board 12.
The tab film 15, as shown in FIGS. 10 and 11, is formed to be a thin sheet-like quadrangular shape having an approximately same 4 sides dimension as that of the base plate 14. An electrode pattern 15a is provided on an upper surface, opposite to the IC package 13, of the tab film 15, to establish an electrical connection with an array of terminals 13a of the IC package 13.
On a lower (rear) surface, which is opposite to the base plate 14, of the tab film 15, a pin type terminal 15b, which is designed to connect to the base plate 14, is provided. And a conductor is provided to the tab film 15 for electrically connecting the pin type terminal 15b and the electrode pattern 15a. 
In addition, the pin type terminal 15b is structured to be able to insert into the concave fitting portion 14e of the round pin 14a of the base plate 14. Furthermore, the tab film 15 has also a bolt hole 15c and a positioning hole 15d having the same dimensions as that of holes 14b and 14c of the base plate at the place corresponding to the bolt hole 14b and the positioning hole 14c of the base plate 14.
Still more, as shown in FIGS. 10 to 12, the ball guide 16 is made from an electrically insulating material and has a quadrangular shape having an approximately same dimension as that of a pressing jig body 20 of the pressing jig 17. At a central portion of the ball guide, an opening 16a for positioning a periphery of the array of the solder balls 13a of the IC package 13 is formed. And a bolt hole 15c and a positioning hole 15d of the tab film 15 are provided and a bolt hole 16b and a positioning hole 16c whose position and the size are corresponding to the bolt hole 15c and the positioning hole 15d are formed to the ball guide.
As shown in FIG. 12, the solder balls 13a disposed at an outermost periphery of the IC package 13 is positioned by the opening 16a. 
Still further, as shown in FIG. 11, the pressing jig 17 is mainly comprised of a pressing jig body 20 having a frame like quadrangular shape, and a cover member 21 attached rotatably to the pressing jig body 20 with an axis 22 and urged by a spring 29 in an opening direction of the cover member 21 (clockwise direction in FIG. 11). A pusher member 23 for pushing the IC package 13 is swingingly (oscillatingly, rockingly) attached to the cover member 21 by an axis 30 and urged by a spring 28 toward a direction away from the cover member 21 (downward direction in FIG. 11). Still further, the cover member 21 is provided with an engaging portion 24, and the pressing jig body is provided rotatably with a latch member 25 by an axis 26. The engaging portion 24 can be engaged with the latch member 25 which is urged by a spring 27 in a clockwise direction as shown in FIG. 12 (in an engaging direction).
As shown in FIGS. 11 and 12, the pressing jig body 20, inside of which the quadrangular IC package 13 is inserted, is structured to be mounted on the tab film 15 and has positioning pins 20a, which are fitted into positioning holes 14c, 15d and 16c, is protruded downward from a lower surface portion of the pressing jig body 20. Further, as shown in FIG. 10, the pressing jig body 20 is provided with bolt holes 20b whose locations and dimensions are each corresponding to each of the bolt holes 16b of the ball guide 16.
As shown in FIG. 11, each of the positioning pins 20a is fitted into each of the positioning holes 14c, 15d, 16c of the base plate 14, tab film 15 and ball guide 16, respectively, thereby each member is positioned and assembled in a predetermined positional relationship. And finally the bolts 18 are each inserted, from upper side, into each bolt holes 14b, 15c, 16b, 20b of the foregoing base plate 14, tab film 15, ball guide 16 and pressing jig body 20 respectively, and then the nuts 19 are attached and screwed up to fix these members as overlapped.
In the IC socket of this kind, as shown in FIG. 12, an outer peripheral edge of the IC package 13 is guided and positioned by an inner surface of the quadrangular frame-like pressing jig body 20 and the solder balls 13a of the IC package 13 are also structured to be positioned by the ball guide 16.
The thus positioned IC package 13 is pressed at its peripheral portion of the upper surface by the pusher member 23, as shown in FIG. 11.
However, in such conventional IC package as mentioned above, since the pin type terminals 15b of the tab film 15 are structured to be fitted into the concave fitting portion 14e of the round pin 14a of the base plate 14, it is difficult to produce the round pin 14a having the concave fitting portion 14e and also difficult to attach and detach the pin type terminals 15b into and from the concave fitting portion 14e. Further, in the conventional IC package, since there is a need to fit the round pins 15b into the concave fitting portion 14e, it is necessary to make the diameter of the round pins 14a large, being inconvenient in reducing an arrangement pitch (distance between the neighboring contact pins) of the contact pins and hence in downsizing the whole size of the IC socket.